Answer
See the answer below.
Work Step by Step
Van der Waals equation:
$\left[P+a\left(\dfrac nV\right)^2\right](V-bn)=nRT$
$(P+6.49\ atm)×3.775\ L=122.43\ L.atm$
$P=25.93\ atm$
Ideal gas:
$PV=nRT$
$P=30.61\ atm$
Percent error:
$(30.61-25.93)/25.93×100\%=18\%$
That's a reasonably large error.
